a) Field of the Invention
The invention is directed to a femtosecond laser system for precise working of material and tissue, particularly a laser device for the precise, micrometer-exact working of organic material, preferably an eye.
b) Description of the Related Art
In a valuable contribution to the art, German Patent DE 197 46 483 by the present Applicant describes how macroscopic amounts of material are ablated, vaporized or melted (CO2 laser, Nd:YAG, excimer . . . ) with micrometric precision when working over large surface areas of materials by means of lasers with large spot diameters.
In another valuable contribution to the art, German Patent DE 197 27 573 by the present Applicant describes an algorithm by which a laser beam can be deflected in order to ensure the best possible, most precise working of material.
U.S. Pat. No. 5,656,186 describes a method for working material while simultaneously preventing or minimizing damaging side effects (melting edges, thermal damage, acoustic shock waves, cracking) through the selection of a special pulse duration depending on the material.
The material-working action of the laser is limited to the small spatial area of the laser focus (typically a few μm3) in which the light intensity is high enough to exceed the threshold of optical breakdown. Localizing on the focus volume, the cohesion of the material is destroyed and a cavitation bubble is formed. When the laser focus is directed to a new position for every laser pulse, linear, flat or three-dimensional cut patterns can be generated. The distance between adjacent cavitation bubbles at the conclusion of the operation must approximately correspond to their diameter so that the material can easily be removed mechanically along the cuts.
Existing laser instruments for working materials with femtosecond laser pulses use regenerative amplifiers with repetition rates of up to 15 kHz by which individual pulses of a femtosecond oscillator are amplified. While the oscillator itself only provides pulse energies in the nanojoule range, the pulses can be amplified up to several millijoules by a regenerative amplifier. While these laser sources are suitable for applications with high ablation rates per laser pulse, they are not optimal for the above-described application for precision cuts.
It is known to use lasers of the type mentioned above for refractive cornea surgery. Usual pulse energies are 5 μJ to 10 μJ. Cavitation bubbles with diameters of 10 μm to 30 μm are generated in this way. These bubble dimensions cause a micro-roughness of the generated cut on the same order of magnitude. On the other hand, it is known that a micro-roughness on this order of magnitude allows only unsatisfactory refractive results.
K. König et al., Optics Letters, Vol. 26, No. 11 (2001) describes how cuts in tissue can also be carried out with nanojoule pulses from a femtosecond oscillator. However, due to the fact that an individual laser pulse does not lead to formation of a cavitation bubble but that, rather, a plurality of pulses placed at the same location are needed to generate a cutting action, this method is only suitable for very fine cut shapes on the micrometer scale. This laser source is not suitable for industrial or medical use.